1. Field of the Invention
The field of art to which the claimed invention pertains is solid-bed adsorptive separation. More specifically, the claimed invention relates to a process for the separation of meta-xylene from a feed mixture comprising meta-xylene and the other C.sub.8 aromatics, which process employs a solid adsorbent which selectively removes the meta-xylene from the feed mixture thereby producing a fluid raffinate stream comprising the remaining C.sub.8 aromatics.
2. Description of the Prior Art
Although meta-xylene constitutes about one-half of the total product stream in the current processes for the manufacture of C.sub.8 aromatics (ethylbenzene and ortho-, meta- and para-xylene), no practical process for its high purity recovery has yet been developed. The difficult and involved prior art separation techniques for recovery of meta-xylene include distillation to obtain higher than the eutectic concentration of meta-xylene followed by crystallization, selective sulfonation, and selective chlorination. One commercialized process for recovery of meta-xylene involves the formation of a complex of meta-xylene, hydrofluoric acid and boron trifluoride, which complex is then extracted by a liquid-liquid extraction technique and finally decomposed to recover the meta-xylene product.
Meta-xylene has great commercial utility because it is needed in the manufacture of certain insecticides and isophthalic acid. Thus, in view of this utility on the one hand, and the above difficult and involved separation techniques on the other, there has long existed a great but unfulfilled need for a more efficient separation process enabling the recovery of high purity meta-xylene.
It is well-known in the separation art that certain crystalline aluminosilicates can be used to separate hydrocarbon species from mixtures thereof. The separation of normal paraffins from branched chain paraffins for example can be accomplished by using a type A zeolite which has pore openings from about 3 to about 5 Angstroms. Such a separation process is disclosed in U.S. Pat. Nos. 2,985,589 and 3,201,491. These adsorbents allow a separation based on the physical size differences in the molecules by allowing the smaller or normal hydrocarbons to be passed into the cavities within the zeolitic adsorbent, while excluding the larger or branched chain molecules.
U.S. Pat Nos. 3,265,750 and 3,510,423 for example disclose processes in which larger pore diameter zeolites such as the type X or type Y structured zeolites can be used to separate olefinic hydrocarbons.
In addition to separating hydrocarbon types, the type X or type Y zeolites have also been employed in processes to separate individual hydrocarbon isomers. In the process described in U.S. Pat. No. 3,114,782, for example, a particular zeolite is used as an adsorbent to separate alkyl-trisubstituted benzene; and in U.S. Pat. No. 3,668,267 a particular zeolite is used to separate specific alkyl-substituted naphthalenes.
Because of the commercial importance of para-xylene, the more well-known and extensively used hydrocarbon isomer separation processes are those for separating para-xylene. Para-xylene is used in the manufacture of terephthalic acid which in turn is subsequently employed in the manufacture of various synthetic fibers such as Dacron, a trade-marked product of the duPont Company. In processes described in U.S. Pat. Nos. 3,558,732 and 3,686,342 for example adsorbents comprising particular zeolites are used to separate para-xylene from feed mixtures comprising para-xylene and at least one other xylene isomer by selectively adsorbing para-xylene over the other xylene isomers. In such processes the adsorbents used are para-xylene selective; para-xylene is selectively adsorbed and recovered as an extract component while the rest of the xylenes and ethylbenzenes are all relatively unadsorbed with respect to para-xylene and are recovered as raffinate components.
In the process described in U.S. Pat. No. 3,917,734 ethylbenzene is recovered in high purity from a feed mixture comprising ethylbenzene and xylene isomers. The process basically comprises contacting the feed mixture with an adsorbent comprising calcium exchanged type X or type Y zeolites, selectively adsorbing the xylene isomers, and thereafter recovering ethylbenzene as a raffinate component. The adsorbent employed is thus all-xylene selective rather than para-xylene selective as are the adsorbents used in the para-xylene separation process. The adsorbed xylenes may then be recovered, in one embodiment, by contacting the adsorbent with a desorbent material, preferably comprising toluene, thereby desorbing the xylenes and then withdrawing the desorbed xylenes from the adsorbent. In another embodiment the adsorption and desorption are done continuously in a simulated moving bed countercurrent flow system the operating principles and sequence of which are described in U.S. Pat. No. 2,985,589.
In the process described in U.S. Pat. No. 3,133,126 adsorptive separations of meta-xylene from other isomers of xylene are shown, but in each instance from mixtures containing the meta-xylene and only one other isomers. The process was unable to recover meta-xylene from a mixture of all of the xylene isomers and ethylbenzene.
Thus, in spite of the great need, no adsorptive separation process employing an adsorbent selective for meta-xylene over the other xylene isomers and ethylbenzene has heretofore been discovered. I have made such a discovery. The process of my invention effectively achieves separation of meta-xylene from a mixture of all C.sub.8 aromatics.